Rolling Contact Fatigue

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Description

When two surfaces are in rolling, or combined rolling and sliding contact, stresses are generated at and below the contact surface. Typically, the highest stress is just below the surface and can be high enough to cause the material to develop fatigue cracks. These cracks propagate with each damaging stress cycle, deeper into the material, but also out to the surface, resulting in spallation/detachment of the surface.

The development of fatigue cracks under these loading conditions is referred to as rolling contact fatigue (RCF). RCF failures are commonly observed in bearing components but also in the rail industry, with cracking occurring in both wheels and rails; the Hatfield disaster in October 2000 was a result of RCF of the rails. RCF may be considered to be an unavoidable failure in rolling contacts, in that it will eventually occur. In the case of bearings, this may be so even if the bearing is correctly aligned, lubricated, and not overloaded. In the application of railway rails, RCF may eventually develop unless the wear rate of the contact surfaces is greater than the rate at which cracks initiated and propagate.

Mechanism

RCF may be described as the process of cracking or material removal due to the application of repeated or cyclic contact between two surfaces. The contact, in this case, is specifically related to a rolling mechanism, although sliding between the two rolling elements is also a contributory factor. Loading between rolling elements causes stress at the contact patch in both components and this stress may reach a maximum, not directly at the surface, but just below the surface, in accordance with Hertzian contact stress laws. However, RCF cracks may initiate either at the surface, or sub-surface depending on the contact conditions.

Surface-initiated cracks are often associated with the concentrated loading at the contact patch between the two surfaces. In the context of railway rails, the loading from the wheel is taken through a relatively small area of contact, and which is sufficiently high to cause yielding (plastic deformation) of the surface. The contact conditions in railway rails are complex with a high degree of sliding which facilitates crack initiation at the highly deformed surface. This surface (both the rail and wheel) is then subjected to permanent and irreversible deformation with the passing of each wheel, leading to the initiation of fatigue at the surface, but also material loss by wear. There is then a ‘competition’ between the initiation of fatigue cracks, and their rate of propagation. Wear of the surfaces may remove the embryonic fatigue cracks or keep them at a size where they do not reach a critical depth. Cracks may propagate at shallow angles to the contact surface, bifurcate, return to the surface creating spalling and detachment of material, or turn to propagate rapidly into the section of the component leading to its complete fracture. The path an RCF crack takes is dependent on the stress pattern in the material, which will change according to localised loading, such as directly from the point of contact, bending from remote loading, or residual stresses.

With rolling elements such as bearings, contact conditions differ from that which occurs in rail-wheel interfaces. Ideally, a correctly functioning lubricant will prevent metal-to-metal contact due to hydro-dynamic effects and the surface should be free from damage. However, due to the Hertzian contact stress criteria, the maximum stress developed sub-surface may coincide with microstructural defects inherent in the material. These may be in the form of inclusions or discontinuities in the material, such as bifilms¹. Cracks can propagate parallel to the surface, eventually turning to intersect the surface leading to spalling and detachment of material. They may also bifurcate and turn to propagate into the material thickness.

1. Bifilms are micro- or sub-microscopic discontinuities in the material which remain from when the material was originally produced, or cast. These result from turbulence in the molten metal which cause films of oxides or other similar high melting point compounds on the surface of the metal to become entrapped and fold over on themselves, preventing fusion/mixing of the two molten surfaces

Appearance and Examples

RCF of rolling elements such as bearings may often go undetected until severe damage or catastrophic failure has occurred; bearings may continue to operate with severely damaged surfaces with spalled material causing metal-to metal contact, abrasion and wear, and with large particulates, loading to cause overload fracture. In the early stages of failure, the development of spallation will cause degradation or failure of the hydrodynamic lubrication which may manifest as noise. At low levels this change or increase in noise may be detected either audibly, or using microphones intended for this purpose. At very low levels of damage, condition monitoring of the oil may also identify metallic wear particles in the oil, providing a warning of the early stages of damage.

The images below show the failure by RCF of a bearing. Spallation of the surface of the race and rolling elements can be clearly seen. In addition, severe wear of the race can be observed.

The equipment that the bearing was fitted to had been operated at a loading well within the capacity of both the equipment and the bearings.

However, material analysis revealed that the hardness of the races and the rolling elements was virtually identical, and it was later identified that the bearings were a pattern type and not from a reputable supplier or OEM. Rolling element bearings typically employ marginally harder rolling elements (e.g. approximately 20-50 Vickers hardness/1-2 Rockwell hardness points) compared to the races to allow for preferential wear, which minimises the potential for RCF.

An example of severe loading that caused lubrication breakdown and metal-to metal contact is the bevel gear used in an automotive application, following endurance testing, as shown below. This image shows detachment of material from close to the root of the gear teeth where the tips of the pinion teeth made contact, initiating RCF.

This next image shows the damage in more detail with the spalled surface and heavy scuffing (smear and wear) of the remainder of the face of the tooth. The damage was relatively consistent around the whole of the wheel and pinion teeth.

This image shows the damaged surface of a bearing race. The dark area is caused by the progression of subsurface RCF cracks with the rib features indicating crack progression as the crack turns up to the surface. The transition between the original surface (top light-coloured area) and the fractured surface shows sections of material just starting to break away at the same width as the rib features.

Avoiding RCF

The measures taken to avoid RCF will be dependent on the given application. In the case of rolling elements such as bearings, as stated above, RCF may be seen as an inevitable mechanism of degradation and failure. Design of the components to ensure loads and contact stresses are well within design parameters is vital as is the control on the cleanliness of the steels used in their construction. Under these ‘known’ conditions there are predicted failure rates and measures can be taken to mitigate the risks, including timely replacement of bearings within specified service lives. The cleanliness of the steels should be monitored to reduce the occurrence of non-metallic inclusions (a known source for fatigue crack initiation) to below specified levels. However, if service conditions change, either intended (e.g. changes in use or modifications to equipment) or unintended (e.g. overloading, malfunctioning of another part of the system, vibrations from imbalanced shafts, distortion, or misalignment) loads on bearings can increase to levels that cause their premature failure.

The examination of bearings removed from service may provide useful information on their operating conditions. This may be carried out on those removed on expiry of their service life (i.e. before failure and at a specified replacement interval) or those removed due to some failure in the system (either of the bearings themselves or another component where contamination may have damaged the bearings for example). Clearly, if the bearings are heavily damaged or destroyed, the evidence of early-stage degradation or failure mechanisms, may be destroyed or at least heavily degraded. However, bearings with minimal or early-stage degradation can yield information on their operating conditions such as loading and rotational conditions (e.g. unintended axial loading on a ball bearing).

The occurrence of RCF in railway rails is well documented and as previously stated, measures are taken to manage RCF and prevent catastrophic failures. The general method of managing RCF in rails is the regular grinding of the rail head to remove the early stages of cracking. Wear rates may be high enough to keep pace with cracking but there is a balance to be struck between cracking and wear rates, the properties of the rail (i.e. different rail grades exhibit variations in hardness and mechanical properties, and hence propensity to fatigue cracking), and intervention with grinding.